In hydraulic brake systems for motor vehicles, it is a usual practice to provide a supply of substantially non-compressible brake fluid as well as structure for applying pressure to the brake fluid so as to feed the fluid to the vehicle brakes. Hydraulic circuits are also utilized in the transmission system of vehicles. Pumps are used both in the brake system and transmission system for controlling the flow and pressure of the fluid in the respective systems. There have been efforts to integrate portions of the hydraulic system such as, for example, the power steering and the brake system of a vehicle as shown in U.S. Pat. No. 3,802,746.
It is desirable to integrate the transmission system and brake system of a vehicle. In such a system, transmission fluid can be utilized as a common fluid for both the transmission and brake systems. However, transmission fluid when aerated causes undesirable pump wear. An additional problem in an integrated brake-transmission system is the use of reservoir tanks in the hydraulic system which may contain a large amount of air, particularly when the amount of hydraulic liquid received in the reservoir tank is decreased. Air in the reservoir will be sucked into the hydraulic system, thereby causing cavitation of the pump. Therefore, reservoir tanks in vehicle hydraulic braking systems have used a closed type construction, in which a flexible diaphragm is provided in the reservoir tank above the level of the liquid received in the tank so as to minimize the amount of air contained in the reservoir tank. The diaphragm may be in actual contact with the liquid. It has been required that the diaphragm freely follow the change in the liquid level without generating any negative pressure. One such type of diaphragm is illustrated in U.S. Pat. No. 4,307,815.
A need has thus developed for a reservoir for use in a brake-transmission hydraulic system in which the reservoir can be maintained adequately filled to inhibit the introduction of contaminants and air into the system while still maintaining the required pressure in the brake-transmission system. A need has further arisen for a reservoir which will prevent aerated fluid from entering the brake system pumps, as well as insuring a sufficient amount of fluid for both the brake and transmission systems.